Messages in this thread | | | Date | Wed, 30 Oct 2013 16:39:31 +0100 | From | Peter Zijlstra <> | Subject | Re: perf events ring buffer memory barrier on powerpc |
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On Wed, Oct 30, 2013 at 04:52:05PM +0200, Victor Kaplansky wrote: > Peter Zijlstra <peterz@infradead.org> wrote on 10/30/2013 01:25:26 PM: > > > Also, I'm not entirely sure on C, that too seems like a dependency, we > > simply cannot read the buffer @tail before we've read the tail itself, > > now can we? Similarly we cannot compare tail to head without having the > > head read completed. > > No, this one we cannot omit, because our problem on consumer side is not > with @tail, which is written exclusively by consumer, but with @head.
Ah indeed, my argument was flawed in that @head is the important part. But we still do a comparison of @tail against @head before we do further reads.
Although I suppose speculative reads are allowed -- they don't have the destructive behaviour speculative writes have -- and thus we could in fact get reorder issues.
But since it is still a dependent load in that we do that @tail vs @head comparison before doing other loads, wouldn't a read_barrier_depends() be sufficient? Or do we still need a complete rmb?
> BTW, it is why you also don't need ACCESS_ONCE() around @tail, but only > around > @head read.
Agreed, the ACCESS_ONCE() around tail is superfluous since we're the one updating tail, so there's no problem with the value changing unexpectedly.
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